This invention relates to polysulfones, and more particularly to impact modification of polysulfone resin. Still more particularly, the high impact strength polysulfones of this invention are polymer blends containing, in addition to a polysulfone, an impact modifier formulation comprising an acrylate core-shell type rubber and a polycarbonate. These blends are characterized by remarkably high impact strengths and an excellent balance of mechanical properties.
Poly(aryl ether) resins, for example, the polysulfone polymers and copolymers derived from Bisphenol A or dihydroxydiphenyl sulfone and 4,4'-dichlorodiphenyl sulfone, are widely accepted for use because of their good strength and thermal properties, and for their stability in a variety of environments. Even though these resins exhibit good impact in un-notched applications, the presence of notching or other surface discontinuities gives rise to stress concentration points and considerably weakens molded parts and extruded goods made from polysulfone. Thus, the impact strengths of polysulfone resins are typically low as measured by mechanical tests such as the notched Izod impact test.
Improvement in impact strength and ductility are needed for better performance of articles fabricated from polysulfones for use in high impact applications and especially those in which the articles may be subject to cracks or have imperfections. Of course, it also is important that such improvements be achieved without substantial adverse effects on desirable properties of the polysulfone. The acceptance of polysulfones over other resins by the compounding art for many applications depends on the resin's excellent stability in a variety of adverse environments, even at elevated temperatures. Methods for increasing resin ductility and impact at the expense of such environmental stability would likely not be readily adopted by the industry.
It also is important that the additives retain their effectiveness after processing and fabricating of the impact-modified polysulfone formulations, as well as during use of articles prepared therefrom.
A variety of impact modifiers are known for use with thermoplastics. Generally, these modifiers are rubbery compositions that can be blended or incorporated into the thermoplastics. Very few rubbery modifiers have been found to be effective for improving impact in polysulfone resins. Those that do, often cause adverse affects losses in other key polysulfone properties. Other impact modifiers that are effective are difficult and/or expensive to produce and thus not available commercially. The impact properties of polysulfone may be improved by using acrylate rubbers, and it is known that a binary blend of a polysulfone and an acrylate copolymer impact modifier has better impact properties than a neat polysulfone. However, these binary blends are also notch sensitive, and the improvement in notched impact strength observed for these blends, if any, is small.
Binary blends of polysulfones and polycarbonates are also well known. Polycarbonates are generally known for their high impact strength properties. When blended with polysulfones, such blends exhibit particularly good hydrolytic stability and are described in the art as having particularly good resistance to environmental stress crazing and cracking. Blends of polysulfones and polycarbonates may thus be useful in a variety of applications where exposure to hot, humid environments is contemplated. However, even though these blends may exhibit an excellent balance of mechanical properties including tensile strength, the addition of a polycarbonate to a polysulfone resin provides little or no improvement in impact properties. The blends are quite notch sensitive and generally have very modest notched impact properties, severely limiting their commercial acceptance.
The good mechanical properties and environmental performance characteristics of polysulfones are thus very important to many applications. Overcoming the low notched impact of polysulfones while maintaining these properties to a high degree, even at elevated temperatures, would widen their commercial utility. Modifiers capable of improving the ductility and particularly the impact characteristics of molded polysulfone articles and extruded goods when used at low concentration levels and further capable of retaining good ductility and low notch sensitivity, particularly over a wide range of temperatures, would give compounders and resin formulators greater flexibility in tailoring the property balance of polysulfone formulations to provide materials well suited for use in a wider variety of applications.
Therefore, there exists a need for a composition which exhibits reduced notch sensitivity, and thereby possesses excellent notched impact strength properties which maintains the other desirable physical and chemical properties of polysulfones.